As methods for rust preventing the surface of a metal, there has been known a zinc or zinc alloy-plating method. However, it is not possible to ensure sufficient corrosion resistance of the metal by such plating alone. For this reason, there has widely been adopted, in this industrial field, the treatment with chromic acid containing hexavalent chromium or the so-called chromate treatment after the plating. Nevertheless, it has recently been pointed out that the hexavalent chromium may adversely affect the human body and the environment and there has correspondingly been such a strong and active trend that the use of hexavalent chromium should be controlled.
As one of the substituent techniques therefor, the formation of a corrosion resistant conversion film, in which trivalent chromium is used, has been known. For instance, Japanese Examined Patent Publication (hereunder referred to as “J.P. KOKOKU”) No. Sho 63-015991 discloses a method, which comprises the step of treating the surface of a metal with a bath containing a mixture of trivalent chromium and a fluoride, an organic acid, an inorganic acid and/or a metal salt such as cobalt sulfate. However, a fluoride is used in this plating bath and therefore, a problem of environmental pollution would arise. In addition, J.P. KOKOKU No. Hei 03-010714 discloses a method, which makes use of a plating bath comprising a mixture of trivalent chromium and an oxidizing agent, an organic acid, an inorganic acid and/or a metal salt such as a cerium salt. However, this method makes use of an oxidizing agent and cerium and therefore, the trivalent chromium may possibly be oxidized into hexavalent chromium, during the processing and/or the storage of the bath.
Japanese Un-Examined Patent Publication (hereunder referred to as “J.P. KOKAI”) No. Hei 10-183364 discloses a method which comprises the step of treating the surface of a metal with a bath containing a phosphoric acid, a salt of metal such as Mo, Cr3+ and Ti, and an oxidizing agent to provide the surface with a hexavalent chromium free, corrosion resistant conversion film. This method uses a large quantity of the oxidizing agent and therefore, the trivalent chromium may possibly be oxidized into a hexavalent chromium, during the processing and/or the storage of the bath.
J.P. KOKAI No. 2000-54157 discloses a method which comprises the step of chemical conversion treating the surface of a metal with a bath containing phosphorus, a metal such as Mo, and trivalent chromium, but no fluoride. However, as a result of our experiments to confirm the effects produced by the method, acceptable corrosion resistance could not be obtained.
Furthermore, J.P. KOKAI No. 2000-509434 discloses a method, which comprises the step of treating the surface of a metal using a plating bath comprising 5 to 100 g/L of trivalent chromium and nitrate residues, an organic acid and/or a metal salt such as a cobalt salt. This method uses, for instance, trivalent chromium in a high concentration and the plating operation is carried out at a high temperature. Therefore, this method is advantageous in that it can form a thick film and ensure good corrosion resistance. However, the method suffers from a problem in that it is difficult to stably form a dense film and that the method cannot ensure the stable corrosion resistance of the resulting film. Moreover, the processing bath contains trivalent chromium in a high concentration and also contains a large amount of an organic acid. This makes the post-treatment of the waste water difficult and results in the formation of a vast quantity of sludge after the processing. Although one can recognize that it is advantageous to use a processing solution free of any hexavalent chromium for ensuring the environmental protection, the method suffers from a serious problem in that it may give a new burden to the environment such that the method generates a vast quantity of waste.
In addition, there could be obtained only films having colorless or an interference color appearances. In this connection, with regard to a black conversion film containing a trivalent chromium on zinc-nickel alloy plating layers (containing 8% or more of nickel in the layers) and zinc-iron alloy plating layers, U.S. Pat. No. 5,415,702 discloses a method which comprises the step of treating the surface of a metal with an acidic bath containing a phosphate compound and trivalent chromium. Also, with regard to a chemical conversion interference color film containing a trivalent chromium on zinc-nickel alloy plating layers (containing 8% or more of nickel in the layers), U.S. Pat. No. 5,407,749 discloses a method, which comprises the step of treating the surface of a metal with an acidic bath similar to that disclosed in U.S. Pat. No. 5,415,702 containing a phosphorus compound, trivalent chromium and oxy-halogen acid ions.
However, in many cases, a Ni eutectoid rate in zinc-nickel alloy plating layers actually produced is less than 8%, and thus it is difficult to obtain a black feature from a practical standpoint. Furthermore, the black conversion film on zinc-iron alloy plating layers does not have enough corrosion resistivity.
Moreover, there have been proposed a method for processing the surface of a metal with a bath containing trivalent chromium in a low concentration and an organic acid and a metal salt such as a nickel salt (U.S. Pat. No. 4,578,122) and a processing method, which makes use of a bath containing trivalent chromium in a low concentration and an organic acid (U.S. Pat. No. 5,368,655). However, these methods never ensure sufficient corrosion resistance of the resulting film as compared with the conventional hexavalent chromate treatment.
It is thus an object of the present invention to provide a processing solution used for forming a hexavalent chromium free, black conversion film, which is applied onto the surface of zinc or zinc alloy plating layers, and which has corrosion resistance identical to or higher than that achieved by the conventional hexavalent chromium-containing conversion film.
Another object of the present invention is to provide a method for forming such a hexavalent chromium free, black conversion film.